[unittests] Remove reverse iteration tests which use pointer-like keys

Summary: The expected order of pointer-like keys is hash-function-dependent which in turn depends on the platform/environment. Need to come up with a better way to test reverse iteration of containers with pointer-like keys.

Reviewers: dblaikie, mehdi_amini, efriedma, mgrang

Reviewed By: mgrang

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D37128

llvm-svn: 311741